Boris Johnson’s plans to build a new airport in the Thames estuary could weaken a legal challenge against expansion of Heathrow he is helping to fund, it was claimed today.

Stephen Knight, a representative from the 2M coalition of 22 local authorities opposed to a third runway at Heathrow, said a legal challenge based on the detrimental environmental impact could be undermined by moves to increase flight capacity elsewhere in the south-east.

Johnson has already pledged £15,000 of Londoners’ money to help the 2M group challenge the government’s Heathrow decision in the courts.

The mayor is nevertheless keen to build another airport on the Thames estuary in the the belief that there is an economic case for increasing capacity in the London area.

Knight said the best way to argue against the expansion of Heathrow on environmental grounds was to propose alternative modes of transport, such as a rail hub around Heathrow and high-speed rail links to northern England, rather than suggest creating extra capacity elsewhere.

Knight told the committee: “One of the biggest elements of our case against expansion is the environmental impact of extra air travel. If we are going to be successful in making that case then obviously we are not going to propose putting it somewhere else. It is our belief that the world as a whole cannot accept an extra three million tonnes of C02 being emitted … A lot of our efforts is looking at alternatives to air travel.

“We clearly welcome the mayor joining us in opposing the third runway but we cannot accept there is a need for extra airport capacity elsewhere.”

Asked about expansion at other airports, such as Gatwick or Stansted, Knight made clear to the committee that talk of increasing flight capacity would undermine the environmental case.

He said: “We clearly welcome the mayor joining us in opposing the third runway but we cannot accept there is a need for extra airport capacity elsewhere.”

Johnson’s director for transport, Kulveer Ranger, said that the “background music” of high-speed rail links to supplant short-haul flights would not address a growing demand for international flights.

He said: “The French realised that when they built their high-speed rail that there was still a need for flights. We need to be realistic.”

Ranger told the assembly panel that the mayor was “mortified” when the government gave the go-ahead to Heathrow expansion last month, given the strength of opposition over environmental concerns for London and quality of life issues for local residents.

A third runway would be a “ticking time bomb” for noise, traffic and air pollution for Londoners, he said.

But he defended the mayor’s plans for a new airport that would allow flights to take off over water.

“It is the job of the mayor to propose what is best for London,” he said.

John Stewart of Hacan ClearSkies, a campaign group opposed to airport expansion, told the assembly he did not believe Heathrow expansion was a “done deal” anyway.

He said it would take 18 months to two years for BAA to draw up its application for expansion, “which is critical because it will be the next government who will give permission or otherwise to BAA”.

Stewart told the panel that Hacan were looking to employ someone for the next 18 months whose job would be to lobby the city and put across that the expansion of Heathrow was not essential to economic growth.